It is a known technique to form a solar panel by depositing an array of silicon spheres over a metal substrate, then processing the silicon spheres to form diodes that convert sunlight to electricity, as described in U.S. Pat. No. 8,133,768.
U.S. Pat. No. 5,556,791 describes a process for forming such silicon spheres by first forming a fine powder of silicon, such as by grinding bulk silicon. This dry powder is referred to as feedstock. Next, a template (a stencil) having small holes formed through it is positioned on a conveyor belt. The holes are then completely filled with the feedstock. The template is then lifted to form uniform piles of the feedstock on the conveyor belt. The piles are then heated by an optical furnace which melts the silicon powder in each pile, and the melted silicon generally forms spheres by surface tension. The spheres are then cooled. The silicon spheres are then further processed and deposited on a substrate to form a solar panel. The silicon spheres have diameters of about 30 mils (762 um).
By using smaller spheres, such as less than 200 um, as a monolayer in a solar panel, there is a very high surface area to volume ratio, making the solar panel more cost effective. It is also beneficial during processing for the spheres to have the same size and electrical characteristics.
It would be extremely difficult to form uniform silicon spheres with diameters less than 200 um using the above-described process. For example, completely and uniformly filling 200 um or smaller holes in a stencil with dry silicon powder would be very difficult due to the frictional forces involved and the variable air gaps between the particles. Although laser drilling of holes less than 200 um to form a stencil is possible, creating a stencil with a sufficiently large number of laser drilled holes in close proximity (e.g., over a million) to make the sphere fabrication cost-effective would be impractical.
A known method for forming silicon spheres less than 200 um in diameter uses sieves to select milled silicon particles within a desired range of sizes, and then a high temperature plasma is used to melt the individual particles in a reducing atmosphere to spherodize the silicon (see U.S. Pat. No. 6,780,219). This process wastes silicon, the extreme heat of the plasma (>2000 C) creates relatively defective spheres, the silicon dioxide spheres are created as a byproduct, and the process is highly energy intensive.
Another known method, described in EP2182555A1, forms silicon spheres of about 1 mm in diameter for solar cells. This process requires sieving roughly shaped silicon powder to obtain particles within a desired range, using a binder to form larger silicon granules generally the size and shape of the desired spheres by means of a tumbling granulator, and placing each individual granule in its own recess of a heating substrate. Conventional heating furnaces, such as resistive or inductive furnaces, are then used to melt the silicon powder in each granule, and the granules form spheres. This process is energy intensive, requires time for a quartz substrate to react with molten silicon, requires a time-consuming process of placing individual granules in recesses, has a limited throughput of about 20,000 granules per substrate, and reduces the longevity of the substrate.
What is needed is a cost-effective method for forming precision silicon spheres having diameters less than 200 um using a heating process that does not compromise the substrate.